Simplify StatisticsManagerActivator/StatisticsProvider contract

Get rid of the need for the StatisticsManagerActivator to keep around
the Provider session -- it is only ever referenced from
StatisticsProvider.start(). Same goes for the various services which
are set into it.

Change-Id: I8014dbdd03e8830bb929e083672f988504d02994
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Simplify StatisticsManagerActivator/StatisticsProvider contract"